1. Field of the Invention
The present invention relates to an optical semiconductor apparatus or device, such as an integrated type coherent detecting apparatus for a coherent optical communication or transmission and the like, a method of driving the optical semiconductor apparatus and an optical transmission or communication system using the optical semiconductor apparatus.
2. Related Background Art
In recent years, research and development of a coherent optical communication system have been extensively advanced because of improved signal-receiving sensitivity, increased signal multiplicity of wavelength division multiplexing system and other technical advantages. One type of receiving method used in the coherent optical communication is an optical heterodyne detection system. The concept of this heterodyne detection system will be briefly described with reference to FIG. 10.
A beat signal created by a signal light and a local light generated by a local oscillator or laser for local oscillation 55 disposed at a signal receiving side is received by a signal receiving device such as a PIN photodiode (PD) 56, and the received signal is demodulated through an amplifier and a low pass filter 58. The oscillation wavelength of the local light needs to be stably deviated from the wavelength of the signal light by a constant value, and therefore, the wavelength or phase of the local light is stabilized by controlling a current injected into the local oscillating laser diode 55 with a controller 57 using the beat signal. Thus, the wavelength or phase difference between the signal light and the local light is stabilized and set to a constant value. This stabilization is called an automatic frequency control (AFC) or a phase-locked loop (PLL).
Presently, a complicated system is constructed, using a plurality of optical components, in order to carry out such optical heterodyne detection. However, it is critical to develop and realize a small coherent receiving device integrated on a common substrate and having improved stability and reliability, in order to improve and put into practice the coherent optical communication technique.
For example, a device has been presented in which a local oscillating laser 101, a three (3) dB coupler 102, and pin photodiodes 103a and 103b are integrated on a common substrate 100 of InP as shown in FIG. 1 (see Electronics Information Communication Academy of Japan IOE 90-3). In this plane waveguide type apparatus, the coupling between an incident signal light a and a light from the distributed feedback type laser 101 is achieved by the three dB coupler 102. Then, the coupled light is split into two and these split beat signals are respectively received by the pin photodiodes 103a and 103b. The two signals are identical with each other, and one is utilized for detection while the other is utilized for the AFC or PLL.
The above-discussed integrated coherent detector, however, has the following disadvantages:
The first disadvantage relates to the fabrication precision of a portion of the three dB coupler 102 and the length of the waveguide. The waveguide is fabricated by photolithography and etching that are conducted after the growth of semiconductor layers on the substrate 100, and the coupling degree or efficiency of the coupler 102 varies due to fluctuation of the fabrication precision of the waveguide. Thus, the yield of a desired coupling degree decreases. Further, as the length of 1 mm to 2 mm is needed as a coupling length of the coupler 102, the entire size of the apparatus increases. In addition, since optical losses in the waveguide increase, the waveguide needs to be formed with a transparent material to a propagated light. As a result, the waveguide should be fabricated with a different material from those of the laser 101 and the photodiodes 103a and 103b. Consequently, the fabrication process becomes complicated, and coupling losses between the waveguide and the laser 101 and the photodiodes 103a and 103b arise.
Second, when a light transmission line is built in a ring type or bus type network that includes a plurality of nodes, a signal transmitter for a repeater is necessary if a signal light is extracted or drawn out from the transmission line all at one node, and therefore, the structure of a signal receiving portion in the node becomes sophisticated. In the case of the above-discussed integrated coherent detection device, one more coupler has to be arranged to guide merely a part of the signal light to the detector, or a laser for signal transmission has to also be integrated to transmit a signal light to the transmission line through the coupler 102. Thus, it is necessary to integrate more elements on the substrate to obtain such a repeater function, and hence the fabrication process becomes complicated.